Engine Tuning; Atmospheric Conditions And Carburetor Settings; Effect Of Setting Parts In Relation To Throttle Valve; Adjusting The Main Jet - Yamaha YZ250 Owner's Service Manual

TUNING
ENGINE
CARBURETOR SETTING
• The role of fuel is to cool the engine,
and in the case of a 2-stroke en-
gine, to lubricate the engine in addi-
tion to power generation.
Accordingly, if a mixture of air and
fuel is too lean, abnormal combus-
tion will occur, and engine seizure
may result. If the mixture is too rich,
spark plugs will get wet with oil, thus
making it impossible to bring the en-
gine into full play or if the worst
comes to the worst, the engine may
stall.
• The richness of the air-fuel mixture
required for the engine will vary with
atmospheric conditions of the day
and therefore, the settings of the
carburetor must be properly suited
to the atmospheric conditions (air
pressure, humidity and tempera-
ture).
• Finally, the rider himself must make
a test-run and check his machine
for conditions (pick-up of engine
speed, road surface conditions)
and for the discoloration of the
spark plug(s). After taking these
into consideration, he must select
the best possible carburetor set-
tings.
It is advisable to make a note of set-
tings, atmospheric conditions, road
surface condition, lap-time, etc. so
that the memorandum can be used
as a reference useful for future.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
Air
Air Hu- pres-
Mix-
temp midi- sure
ture
. ty (alti-
tude)
Low Rich-
High High
(high) er
High Lean
Low Low
(low) er
The reason for the above tendency is
that the richness or leanness of a fuel
mixture depends on the density of the
air (i.e. the concentration of oxygen in
it).
• Higher temperature expands the air
with its resultant reduced density.
• Higher humidity reduces the
amount of oxygen in the air by so
much of the water vapor in the
same air.
• Lower atmospheric pressure (at a
high altitude) reduces the density of
the air.
TEST RUN
After warming up the engine
equipped with the standard type car-
buretor(s) and spark plug(s), run two
or three laps of the circuit and check
the smooth operation of the engine
and discoloration of spark plug(s).
Discoloration
Normal
Over burned
(too lean)
Oil fouled (too
rich)
Set-
ting
A. Normal
B. Over burned (too lean)
Lean
C. Oil fouled (too rich)
er
Rich-
er
7-1
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING
Condition of spark
plug
Insulator is dry
and burnt brown.
Insulator is whit-
ish.
The power jet closes at 8,500 rpm of
Insulator is sooty
the engine, after which only the main
and wet. jet dominates.
A. Closed
B. Full-open
1.
2.
3.
4.
5.
6.
7.
8.

ADJUSTING THE MAIN JET

The richness of air-fuel mixture with
3/4–4/4 throttle can be set by chang-
ing the main jet "1".
Standard main jet
* Except for USA and CDN
1. Spark plug is too hot.
• Select a main jet having higher
calibrating No. than standard. (To
be enriched)
2. Spark plug is wet.
• Select a main jet having lower cal-
ibrating No. than standard. (To be
leaned out)
ENGINE
Pilot jet
Pilot air screw
Jet needle
Diameter of straight portion
Clip position
Throttle valve
Power jet
Main jet
#178
*#180
7